Moisture adsorbents such as silica gel and molecular sieves are generally used for removing moisture from gases. Silica gel is a gel of silicic acid having strong adsorptivity and composed of SiO.sub.2 .multidot.nH.sub.2 O. Silica gel is porous and in some cases has a surface area as large as 450 m.sup.2 per gram. The adsorptivity of silica gel depends upon the amount of water contained therein, and the more dehydrated it becomes, the greater its adsorptivity, so long as the gel structure is maintained. FIG. 7 is a graph showing the amounts of the water adsorbed by adsorbents in which the axis of ordinate represents adsorption and the axis of abscissa represents relative humidity. The adsorptivity of silica gel acts within a wider range than that with activated carbon and is superior to activated carbon with respect to such characteristics as incombustibilty and mechanical fastness.
Conventional moisture adsorbents have problems in that the corrosion of various types of adsorbents easily progresses in environments of high temperature and high humidity, and adsorbents such as silica gel have limited moisture adsorptivity and thus cannot remove moisture from a gas once they have reached the saturated adsorption state. In addition, fixed magnetic disk units which are conventionally installed in air-conditioned rooms exclusively used for such purposes have recently begun to be installed in more general environments, resulting in the need to remove with high reliability the moisture contained in these units.